How did the tower of Pisa remain unharmed from earthquakes throughout the years?

The Tower of Pisa, a 58 meter-tall campanile situated behind the Cathedral of the Italian city of Pisa, is known worldwide for its tilting. In fact, prior to restoration work performed between 1990 and 2001, the tower leaned at an angle of 5.5 degrees, which was minimized to about 3.99 degrees. Its construction began in 1173 and it has since managed to survive, undamaged, at least four strong earthquakes that have hit the (seismically active) region after 1280. To those wondering how this was even possible, a team of engineers can now give the answer, as they recently concluded that the survival of the tower can be attributed to a phenomenon known as dynamic soil-structure interaction (DSSI). The results of their study have been presented to international workshops and will be formally announced at the 16th European Conference in Earthquake Engineering taking place in Thessaloniki, Greece next month [18 to 21 June 2018].

The Leaning Tower of Pisa is the third oldest structure in the city's Cathedral Square, after the cathedral and the Pisa Baptistery – both of which are also sinking. It reportedly started to lean when builders reached the third story, as the ground was too soft on one side to properly support the structure's weight. The tilt increased in the decades before the structure was completed in 1370, and continued to increase until the structure was stabilized during the 10-years restoration work, when the severity of the issue was gradually reduced.

The 16-member research team, led by Professor Camillo Nuti from Roma Tre University, studied available seismological, geotechnical and structural data records over time, and analyzed the physical, mechanical, and chemical properties of the materials used to build the tower. Also taking into account the soil in which it was anchored, it turned out that the tower’s height, combined with the softness of the foundation soil and the structural strength of its marble all contributed to its remarkable resilience, as the structure does not resonate with earthquake ground motion. The combination of these characteristics gives the Tower of Pisa the world record in DSSI effects. Professor Mylonakis, Chair in Geotechnics and Soil-Structure Interaction, and Head of Earthquake and Geotechnical Engineering Research Group in the Department of Civil Engineering at the University of Bristol, said: "Ironically, the very same soil that caused the leaning instability and brought the Tower to the verge of collapse, can be credited for helping it survive these seismic events."